Among the various techniques to which the present invention can be applied, the interference fitting technique which is applicable to shrink fitting, expansion fitting and press-fitting, and which represents a typical method of combining together solid masses of mainly cylindrical configurations, is widely utilized as a means of mechanically combining two parts together. In utilizing this technique, the acquisition of the knowledge of the contact stress at the contacting surfaces of two abutting parts to serve as a measure for determining the degree of interference fit is a matter indispensable for knowing the functioning as well as the mechanical strength of a machine, or for knowing the degree of the true interference fit of combined portions of parts. Heretofore, however, there has not been established a method of directly and quantitatively measuring the contact stress at the contacting surfaces of combined parts.
For the reasons stated above, various means of measuring contact stresses are being studied. However, generally, there have been known only the method of indirectly measuring a contact stress by making use of photoelasticity, and also the method of directly measuring the contact stress by utilizing a pressure-sensitive sheet.
The method which utilizes photoelasticity is such that a model of contacting surfaces is prepared in advance with a synthetic resin which differs in material from the object whose contact stress is to be measured actually, and evaluation is made based on the fringe patterns formed photoelastically at the contacting portions of the model. Thus, a uniformity of nature or property between the object for measurement and the model is required. However, actual no uniformity cannot be obtained owing to the difference in the material, dimension and configuration between the object and the model. Accordingly, not only does there result a delicate discrepancy in the result of the measurement, but also the measurement is affected by the degree of machining precision of the contacting surfaces of the model, causing changes in the optical property of the contacting surfaces to arise, leading to a difficulty in determining the degree of the fringe patterns. On the other hand, the method utilizing a pressure-sensitive sheet for measuring contact stress of stationary planar surfaces such as bolt-fastened contacting surfaces requires the pressure-sensitive sheet to be inserted in advance between the contacting surfaces before measurement. As a result, the state and the property of the contacting surfaces could change in some way or other. As such, according to this prior method which includes measuring the contact stress by separating the contacting surfaces and then observing the condition of the pressure-sensitive sheet, it is not possible to avoid lowering of the accuracy of measurement, and also it is not possible to perform quantitative measurement either. Not only that, the prior method cannot be utilized in making a measurement of contact stress at the contacting surfaces of an interference fit such as shrink fit, expansion fit or press-fit surfaces. The prior methods stated above invariably are directed only to the measurement of the stationary contact stresses, and have the inconvenience and drawback that they do not permit measurement of the so-called dynamic contact stresses wherein the mutual contacting surfaces are displacing relative to each other.
In order to overcome the problems of the prior techniques as stated above, and to perform a measurement while maintaining the state and the nature or property of the contacting surfaces which are to be measured, research is being conducted as the use of ultrasonic waves in contact stress measurement. (see Non-Destructive Inspection, vol. 25, 10th issue, pp. 669-675).
This latter research involves the known technique in which only the intensity of the reflecting wave of an ultrasonic wave at the contacting surfaces of two abutting metal masses is measured, to thereby measure the contact stress at such surfaces. With the known techniques of processing metal surfaces, it is not possible to manufacture a true planar surface or a true curved surface which is free of surface roughness or windings. Rather, usual contact surfaces are inevitably constructed with two abutting surfaces having surface roughness or windings. Such a contact is microscopically enlarged and shown schematically in FIG. 1. It will be noted therein that the two contact surfaces are comprised of a true contact portion C wherein two metal masses are in direct contact with each other, and a contact portion N wherein the two metal masses are not in direct contact with each other and there is air therebetween. When an ultrasonic wave is directed toward the contacting surfaces, there is a tendency for the intensity of the reflecting wave to become smaller with an increase in the area of the true contact surfaces. This known stress measurement technique makes use of said tendency. In fact, the measurement of contact stress is based on this tendency.
In a concrete method of making a measurement which has been attempted in said research, reference test pieces are used to preliminarily establish the reference relationship between contact stress and intensity of reflecting wave, and using this relationship as a correction curve, quantification of the contact stress on the basis of the reflecting wave intensity measurement value is attempted. It has been reported, however, to the effect that "at the present stage, the accuracy of measurement cannot be said good enough to put the present method to practice".
The above-stated method of measuring the intensity of the reflecting wave of ultrasonic wave at the contacting surfaces in order to determine the contact stress between two abutting metal masses is theoretically possible. According to the research conducted by the present inventors, however, it has been found that this proposed prior technique bears various shortcomings from the practical point of view. More particularly, FIG. 2 is an explanatory illustration showing the manner of measuring the intensity of the reflecting wave at the contacting surfaces. In FIG. 2, I and II represent carbon steel pieces, respectively, for machine structural use. These steel pieces are made of a material labeled S35C (JIS G 4051, but containing 0.32-0.38% of carbon). The metal piece I has a thickness t.sub.1 =30 mm, and the metal piece II has a thickness t.sub.2 =20 mm. An ultrasonic wave sensor 2 is applied to a surface of the metal piece I, and an ultrasonic wave having a frequency of 5 MHz is directed onto the contacting surfaces in a direction normal thereto from the sensor 2. While varying the contact stress .sigma. of the metal pieces I and II, the intensity h(dB) of the reflecting wave of the ultrasonic wave is measured by the sensor 2. The result of this measurement is shown by way of a graph in FIG. 3. It should be understood that this measurement was conducted by using an ultrasonic flaw detector of pulse echo type, so that the result of the measurement represents the value indicated in decibels on the Braun tube display screen (CRT) of the apparatus.
As will be noted from FIG. 3, the variation of the intensity of the reflecting wave due to the increase in the contact stress is very small, being about 0.2 dB/kg/mm.sup.2. Such a magnitude of variation as this constitutes a big obstacle in making a correct quantitative evaluation of the contact stress relative to the intensity of the reflecting wave. Furthermore, as stated above, this method preliminarily establishes a reference relationship between the contact stress and the intensity of reflecting waves, and evaluation is made by comparison between this reference value and the intensity of the reflecting wave which is the result of the measurement. It should be noted, however, that with respect to the external surface of the reference test piece on which the ultrasonic wave sensor is placed and also to the external surface of the object requiring a measurement of the stress at the contacting surfaces thereof, there inevitably exist differences in the shape and roughness of the external surfaces, and a difference in the manner of applying the ultrasonic wave thereonto. These differences directly affect the result of the measurement, causing variance in the result of the measurement, and thus making the quantitative evaluation all the more difficult.